The purpose of this research study is to investigate the relationship between blood flow and oxygen delivery to the brain upon memory in men and women between 55 and 84 years of age. It will also study whether an aerobic exercise program consisting of walking and indoor cycling can improve brain function. Falls contribute substantially to major medical costs and suffering in the elderly. One-third of older persons living in the community will experience a fall, and declining memory has been identified as one of the major risk factors for falling. If exercise such as walking or cycling can improve brain function and memory, it may be possible to reduce fall risk in older adults by regular participation in a moderate aerobic exercise program.